1. Field of the Invention
This invention relates to an apparatus for receiving reflected ultrasonic waves for use in ultrasonographic systems, and more particularly to such an apparatus employing a novel filter arrangement.
2. Description of the Prior Art
Ultrasonography utilizes ultrasonic waves to visualize various body structures for diagnosis. Ultrasonic waves, once they enter a test body, such as a living human body, undergo a relatively large degree of attenuation, to the extent that ultrasonic energy losses are not negligible for proper reception of reflected ultrasonic waves. In a form of ultrasonography, wherein echoes of ultrasonic pulses are received, an echo, reflected from the closest area and subjected to the least attenuation, while ultrasonic energy goes into the body and reflected back, is first received, and pulse echoes from more remote areas are successively received which are increasingly attenuated with increased distance from the receiver. To cope with this problem, there has been practiced in the art a system known as the TGC system, wherein the gain of an echo signal receiver is first reduced and progressively increased as time elapses, from the time the first ultrasonic pulses are emitted, while reflected ultrasonic waves are being received.
However, the known TGC echo reception system is disadvantageous in that it cannot compensate for energy dispersion in the medium, such as the human body. More specifically, weak echo signals, that is, echo signals which are of a low level or are reflected from deeper areas, cannot fully be received simply by increasing the gain of an amplifier. It is preferable that areas, which reflect weak ultrasonic waves, as viewed from a probe, be observed with lower frequencies, taking into consideration, reflective characteristics of such areas and dispersion in the medium being tested, which cause greater energy losses at higher frequencies.
One known system designed to meet such requirements, is for example, disclosed in U.S. Pat. No. 4,016,750. In this system, reflected ultrasonic waves are received by a receiver having a filter with its central frequency fo progressively lowered as the receiver receives echo signals from reflecting areas increasingly remote, or the time required to receive echo signals becomes increasingly longer. This system is effective for use with an object constituted by reflecting areas or sources that send back echoes which can be received at levels which become lower as the distance to the reflecting sources is greater. However, actual objects or bodies to be tested contain various tissues which have different attentuation characteristics, dispersing properties, and reflecting properties. Thus, this reception system is not considered effective for use with such objects which provide no constant or linear relationship between the distance to reflecting areas and the level at which reflected ultrasonic waves are received, such as the living human body.
The difficulties with the prior system can be shown with reference to FIG. 1, which depicts an ultrasonographic system, wherein ultrasonic waves are emitted into and received from a test body 3 by a probe 1 with a transitional device, such as water bag 2, interposed therebetween. In this system, the probe 1 sends ultrasonic waves into the body 3, and parts of such body as scanned with reflect back the ultrasonic waves, such as by echo signals, and probe 1 will receive such echo signals. With such an ultrasonographic system, the probe 1 tends to receive lower components of ultrasonic waves, as reflected from the test body 3, when the central frequency of the filter is lowered, as echo reflecting areas become deeper and further from the probe. Accordingly, no good display quality is available since the lower the cutoff frequency, the better the quality of displayed images. In other words, in such a system for ultrasonic echo imaging, the system receives a much lower frequency than when a water bag is not used and actual echo from the object human being body comes back. This is due to the system center frequency being tuned down monotonically at a predetermined rate without any regard to the actual situation of the echo sources. Thus, only poor quality sonic image is available, since in general, the more the lower frequency component is suppressed, the better the image quality (especially in resolution and detailed texture) is to be obtained.